Sustainable cool pigments based on iron and tungsten co-doped lanthanum cerium oxide with high NIR reflectance for energy saving

Dyes and Pigments 154 (2018) 1–7

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Sustainable cool pigments based on iron and tungsten co-doped lanthanum cerium oxide with high NIR reflectance for energy saving

T

Jinqing Chena, Yu Xiaoa,b, Bin Huangb, Xiaoqi Sunb,∗ a

School of Metallurgy and Chemical Engineering, Jiangxi University of Science & Technology, Ganzhou, 341000, China CAS Key Laboratory of Design and Assembly of Functional Nanostructures, and Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China b

A R T I C L E I N F O

A B S T R A C T

Keywords: La2Ce2O7 Sustainable Cool pigment Near-infrared reflectance

The novel non-toxic yellow-orange inorganic pigments La2Ce2-xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7, 0.9) with high near-infrared reflectance have been synthesized. The crystal structure, chemical composition, optical properties (reflectance and absorption properties) and color performance of the synthesized pigments were characterized by several techniques. X-ray diffraction analysis indicated the disordered defect fluorite-type structure of the pigments. The typical pigment La2Ce1.5W0.25Fe0.25O7+δ possessed the high NIR solar reflectance of 82.05% with orange color (L* = 77.21, a* = 8.79, b* = 35.80). Also the influence of calcination temperature has been studied, the higher temperature resulted in enhanced color and slightly reduced reflectance properties. The applicability of typical pigment La2Ce1.7W0.15Fe0.15O7+δ (calcined at 1150 °C) was tested on a galvanized sheet, the coating improved the NIR solar reflectance from 19.18% to 71.07%. The acid/alkali resistance studies confirmed the pigments were chemically stable. Thus, the synthesized pigments with vivid yellow-orange color and high NIR reflectance have exhibited application potentials.

1. Introduction The sun played an important role to human being on earth, it provided light and warmth for us to survive. Also the solar energy heated the objects, in some scenes such as buildings and cars, this additional energy resulted in rising temperature of interior area and leading to extra energy consumption for air conditioning [1–5]. What was worse, the urban heat island effect and global warming caused the temperatures of our living conditions higher than ever before [6–8]. Accordingly, the increase of energy consumption for cooling the interior environment has become a challenge for us. Because of the ultra-high reflectance of solar radiation, the cool pigments borne out to reduce the interior temperature effectively [9]. The solar radiation was reported to be composed of ultraviolet radiation (5%), visible radiation (46%) and infrared radiation (49%) in different wavelength ranges [10]. The infrared radiation brought to be the major heating energy, which transferred most of the energy to building. Cool pigment possessed the properties of absorbing less solar energy in infrared radiation and keeping the lower temperature of object surface, which transferred less energy to the interior space and saved energy for air conditioning [11]. Literature has given the significantly difference between NIR solar reflectance pigment and ordinary pigment, i.e., the interior temperature

∗

difference of boxes coated with cool pigment LaFe1-xAlxO3 and ordinary pigment was indicated to be 5.4 °C [12]; A tile coated with a new glass ceramic material provided a great energy saving over 20% in comparison with conventional ceramic tiles, the interior temperature of boxes coated with cool pigment was obviously lower than ordinary ceramic tiles [13]. Therefore, the use of cool pigment has been an effective method to save energy for air conditioning consumption. Although white coatings possessed a high NIR solar reflectance (> 80%), most people tended to use colored coatings to decorate buildings and cars. Unfortunately, the traditional colored pigments absorbed more infrared radiation and part of them contained heavy metal elements, such as chromium, lead, cobalt, cadmium and antimony. So researchers have been committed to develop the non-toxic pigments with colorful hue and high NIR solar reflectance [14]. From literature, various yellow pigments were reported, i.e., Mo-doped cerium gadolinium oxide [15], Cr2O3-3TiO2 orange nanopigment [16], enhanced BiVO4 [10,17] and Mo doped Y2Ce2O7 [18]. These pigments were environment-friend enough with high NIR reflectance to replace the use of traditional toxic heavy metal yellow pigment, such as cadmium yellow and lead antimoniate (PbSbO3) [19]. Among these pigments, the color of enhanced bismuth vanadate was richer than the others, but the raw material was expensive which limited its

Corresponding author. E-mail address: [email protected] (X. Sun).

https://doi.org/10.1016/j.dyepig.2018.02.032 Received 2 November 2017; Received in revised form 4 February 2018; Accepted 19 February 2018 0143-7208/ © 2018 Published by Elsevier Ltd.

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mapping of the samples were analyzed using the EMAX x-act- liquid nitrogen less X-ray detector which was obtained on scanning electron microscopy (S4800, Hitachi, Japan). The UV-vis-NIR spectrophotometer (Agilent carry 5000, America) was used to character the optical properties of pigments and coatings using polytetrafluoroethylene (PTFE) as a white standard. The powder samples were filled in the powder cell (the coatings sheet were directly covered in the equipment) and performed in reflection mode with a resolution of 1 nm and the electromagnetic spectra ranging from 300 nm to 2500 nm. The NIR solar reflectance (R*) in the wavelength ranging from 800 nm to 2500 nm of the pigments and coatings were calculated by the following formula according to ASTM G173-03:

application. Recently, many researchers, including our group, have developed rare earth based environment benign pigments with color diversities [20–23]. The use of rare earth oxides was due to their excellent chemical and thermal stabilities, low toxicities, together with unique spectroscopic and magnetic properties [24]. Among the rare earth pigments, CeO2 has been applied very frequently in recent studies, including Y2Ce2O7 [18], Sm2Ce2O7 [25], GdCeO3.5 [15] and so on. Cerium oxide and other rare earth oxides could form stable crystal structures. The molybdenum was doped to these pigments, and exhibited good yellow colors with high NIR solar reflectance. In our recent study, the Tb/Pr doped La2Ce2O7 red-orange pigments have been prepared, which possessed high NIR reflectance even with dark orange color [26]. In this article, the Fe3+ and W6+ were co-doped to lanthanum cerium mixed oxide (La2Ce2O7) to prepare a new series of yellow pigments with high NIR reflectance for energy saving.

R∗ =

2500 ∫700 r (λ ) i (λ ) dλ 2500 ∫700 i (λ ) dλ

where r(λ) is the spectral reflectance obtained from the UV-vis-NIR spectrophotometer and the i(λ) is the standard solar spectral irradiance from ASTM G173-03 Reference Spectra (W*m−2*nm−1) [27]. The band gap was measured using absorbance spectra ranging from 300 nm to 800 nm and calculated from the absorption edge by the equation E (ev) = h·c/λ = 1239 (eV nm)/λ (nm). The color performance of synthesized pigment was obtained using the spectrophotometer CS-580A (Hangzhou CHN Spec) with CLEDs light source using CIE L*a*b* (1976) color space system to evaluate color property which was recommended by the Commission Internationale de l’Eclairage (CIE). L* indicates the brightness value (black = 0, white = 100), a* for red (+) and green (−), and b* represents the yellow (+) and blue (−) value. Furthermore, the L*C*H° color model uses the same color space as L*a*b* system. The saturation of the color is defined as C* = [(a*)2+(b*)2]1/2, and H° = tan−1(b*/ a*) indicates the hue angle, H° = 35°–70° for orange, H° = 70–105° for yellow.

2. Materials and methods 2.1. Reagents and materials La(NO3)3·6H2O, Ce(NO3)3·6H2O, Fe(NO3)3·9H2O, (NH4)6H2W12 O40·xH2O and ethylene glycol with analytical grade (99% purity) were obtained from Aladding Reagent (Shanghai) Co. The citric acid was provided by Shandong Xiya chemical technology Co. Ltd. All the starting materials were not further purified before use. 2.2. Synthesis of La2Ce2-xW0.5xFe0.5xO7+δ The pigments of La2Ce2-xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7, 0.9) were prepared via a Pechini route. Stoichiometric proportions of the raw materials were dissolved in the adequate ethylene glycol corresponding to the above formula. Citric acid in the mole ratio of 2:1 with respect to the cations (La, Ce, W, Fe) was added to the mixture solution as the chelation agent. The mixture was rapidly dissolved using an ultrasonic cleaner until the solution was clear. Subsequently, the clear mixture solution was heated at 80 °C on a thermostatic heating plate to promote the polymerization reaction. A lot of bubbles were generated in the solution after 20 min and lasted for 5 min, then the wet polymerization gel was formed (accompanied by the polymerization, the citric acid and ethylene glycol esterification were reacted to produce esters and water). The wet gel was then transferred into a vacuum oven and dried for 10 h at 80 °C, and the obtained xerogel precursor was grounded into powders. Finally, the pigments were obtained by calcining at 1000 °C, 1150 °C and 1300 °C for 10 h, respectively. The amount of raw materials has been listed in a Table 1.

3. Results and discussion 3.1. Powder X-ray diffraction analysis Fig. 1a shows the powder X-ray diffraction patterns of the sample La2Ce1.9W0.05Fe0.05O7+δ calcined at 1000 °C, 1150 °C and 1300 °C, respectively. XRD curves indicate that the sample are highly crystalline in nature calcined over 1000 °C. All the reflections are indexed as the disordered defect fluorite-type structure with the space group Fm3m, which is the same as previous reports [26,28–30]. The fluorite structure with (111), (200), (220) and (311) planes are corresponding to diffraction peaks of 2θ at 27.92°, 32.34°, 46.26° and 54.78°, respectively. As can be seen from Fig. 1(b), with the increase of calcination temperature, the intensities of characteristic peaks are increased and the peak widths at half height are narrowed. No other phase was observed from XRD pattern, indicating that W6+ and Fe3+ entered into the La2Ce2O7 lattice and formed the La2Ce1.9W0.05Fe0.05O7+δ solid solution. But the electroneutrality was not achieved with the entranced W6+ and Fe3+, only one Ce4+ was substituted. Accordingly, the existed cation vacancies in the lattice and the structure were defective. The XRD patterns of La2Ce2-xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7,

2.3. Characterization techniques and instrumentation The crystalline structures of calcined pigment samples were identified by Rigaku Miniflex 600 XRD system using Cu-Kα radiation with continuous scanning mode at a rate of 6°/min and a step size of 0.02° ranging from 5° to 100°. Operating conditions of 40 kV and 15 mA were used to obtain the XRD pattern. Energy dispersive X-ray spectroscopy (EDS) analysis and elemental

Table 1 The amount of raw materials for preparing La2Ce2-xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7, 0.9). x

La(NO3)3·6H2O/g

Ce(NO3)3·6H2O/g

(NH4)6H2W12O40·xH2O/g

Fe(NO3)3·9H2O/g

0.1 0.3 0.5 0.7 0.9

1 1 1 1 1

0.953 0.853 0.752 0.637 0.522

0.015 0.044 0.074 0.103 0.132

0.023 0.070 0.117 0.163 0.209

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3.2. EDS and elemental mapping analysis The chemical composition of resulting pigment was determined using EDS analysis. Fig. 3 present the EDS spectra and X-ray dot mapping of the typical sample La2Ce1.3W0.35Fe0.35O7+δ pigment calcined at 1000 °C. EDS spectra of La2Ce1.3W0.35Fe0.35O7+δ sample shows the presence of La, Ce, W, Fe and O element (all the expected elements), which are close to the theoretical calculation values. X-ray dot mapping of La2Ce1.3W0.35Fe0.7+δ pigment shows that the elements are uniformly distributed in the lattice, which further confirm the homogeneity of formed phases. Thus, the powder X-ray diffraction and EDS analysis confirm that iron and tungsten have been averagely inserted into the La2Ce2O7 lattices and formed uniform lattices. 3.3. Optical properties and color performances Fig. 1. Powder X-ray diffraction patterns of La2Ce1.9W0.05Fe0.05O7+δ pigment calcined at 1000 °C, 1150 °C and 1300 °C. Fig. 1(b) shows the expanded XRD of the peak around 2θ 26.5–29.5°.

The absorption spectra and diffuse reflectance spectra in the visible region of the La2Ce2-xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7, 0.9) pigments calcined at 1000 °C are shown in Fig. 4. In order to observe the introduction of new energy bands associated to dopants, the spectrum for undoped sample x = 0 has also been added in Fig. 4. A strong absorption around 410 nm in the UV-vis reflectance spectrum is due to the charge transfer transition between O2p valence band and Ce4f conduction band of Ce4+. The replaced W6+ for Ce4+ resulted in the cation vacancies, which caused the defection of lattice. The lattice defects cause the pigment to absorb light at wavelengths of 450 and 520 nm, resulting in a yellow-orange color of the pigment [32]. It is worth mentioning that the diffuse reflectance curves of the La2Ce2xW0.5xFe0.5xO7+δ (x = 0.3, 0.5, 0.7, 0.9) compounds clearly exhibit an additional broad absorption peaks or bands around 700 nm. These absorptions probably due to the Fe3+ d-d intra-atomic transitions [33]. The doping of W6+ for Ce4+ in La2Ce2O7 shifts the absorption edge to the low wavelength, and as a result the band gap decreases from 3.08 to 2.84eV (see Table 3). The addition of W5d orbitals above the Ce4f orbitals results in the widening of conduction band, this leads to the reduced interaction between O2p and Ce4f orbitals, which in turn increases the band gap [15]. Therefore, the colors of pigment samples change from ivory-yellow to orange. With the dopant increasing, the band gap increase gradually as shown in Table 3. The band gap of the sample falls in violet region, so the yellow-orange color can be observed. In accordance with the ASTM standard G173-03, the NIR solar reflectance spectra of La2Ce2-xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7, 0.9) orange pigments calcined at 1000 °C are illustrated in Fig. 5 (the NIR reflectance spectra inset), and the corresponding detailed information of NIR reflectance spectra and NIR solar reflectance spectra are given in Table 3. As can be seen from the NIR reflectance spectra, the La2Ce1.9W0.05Fe0.05O7+δ samples possess the average NIR reflectance of 83.29%. With the increasing of co-doped W6+ and Fe3+ for Ce4+ in La2Ce2O7, the NIR reflectance is gradually uplifted to 88.10% when 12.5% (W, Fe) was doped, then slightly decreases to 85.45% with more dopants. The above results can be attributed to the co-dope of W6+ and Fe3+. As we explored, the doped Fe3+ would bring brown color hue to the pigment, but the color was dim and violently decreased the reflectance in NIR range. Hence, the W6+ was added to enhance the lightness of pigment and improve the NIR reflectance. As the corresponding NIR solar reflectance, the successive doping of W6+ and Fe3+ improves the NIR solar reflectance from 78.22% (x = 0.1) up to 82.05% (x = 0.5), further increase the dopant slightly reduce the reflectance to 78.61% (x = 0.9). Moreover, the NIR reflectance of the yellow-orange pigment is found to be higher than that of Fe-doped MgTiO3 orange pigment (R* > 53.51%, L* = 74.61, a* = 13.92, b* = 23.37), LaFeO3 (R* = 52.5%, L* = 64.69%, a* = 10.28, b* = 24.59) and Cr2O3-3TiO2

Fig. 2. Powder X-ray diffraction patterns of La2Ce2-xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7, 0.9) pigments calcined at 1000 °C. Fig. 2(b) shows the expanded XRD of the peak around 2θ 27.7–28.4°.

Table 2 The lattice parameter of La2Ce2-xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7, 0.9) orange pigment calcined at 1000 °C. Sample

x = 0.1

x = 0.3

x = 0.5

x = 0.7

x = 0.9

Lattice parameter (Å)

55537

5.5440

5.5372

5.5309

5.5117

0.9) pigments calcined at 1000 °C are given in Fig. 2. As the contents of iron and tungsten are increased, the samples keep the disordered defect fluorite-type structure unchanged. The additional peaks in pattern (x = 0.9) can be probably attributed to the overmuch dopant in lattice, which corresponding to the mixture of FeLaO3 (JCPDS Card No. 371493) and Fe2O3 (JCPDS Card No. 54-0489). Fig. 2b shows the zoom-in part of the range of 2theta from 26.5 to 29.5°. The lines in Fig. 2b indicate the shift inclination of (111) peaks toward higher angle side with the increase of doped amount. Considering the radii of La3+(0.1116 8CN), Ce4+(0.097 nm 8CN), Fe3+(0.078 nm 8CN) and W6+(0.06 nm 6CN), the shifting effect of diffraction peaks should be ascribed to the lattice shrinkage caused by the increase of smaller Fe3+ and W6+ ion concentration [31]. The lattice parameter of sample was calculated with jade6.5 software and list in Table 2, it is clear that the lattice parameter of La2Ce2-xW0.5xFe0.5xO7+δ were decreased with more dopant.

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Fig. 3. EDS spectra and X-ray dot mapping of La2Ce1.3W0.35Fe0.35O7 pigment calcined at 1150 °C.

Table 3 L*, a*, b* color coordinates, band gap energy, and NIR solar reflectance for the La2Ce2xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7, 0.9) pigments calcined at 1000 °C. Sample (1000 °C)

L*

a*

b*

C*

H°

Eg (ev)

R%

Solar R*%

0.1 0.3 0.5 0.7 0.9

76.43 76.21 77.21 76.53 73.16

2.01 5.43 8.79 10.78 12.39

23.39 31.53 35.80 37.36 37.81

23.47 31.99 36.86 38.88 39.79

85.09 80.23 76.21 73.90 71.86

3.08 3.06 3.05 3.03 2.84

83.29 87.14 88.10 86.00 85.45

78.22 81.79 82.05 80.59 78.61

L* = 94.18, a* = -1.26, b* = 9.93) have been reported in our recent study [26]. The co-dopant of W6+ and Fe3+ increase a* value of the pigment from 2.01 to 12.39, meaning that the redness hue of prepared pigment has been strengthen. The b* value gradually increases with the more dopant concentration from 23.39 to 37.81, hence the yellowness of the sample is enhanced. It worthwhile to mention that the co-doped W6+ and Fe3+ cannot improve the b* value of pigment when their doped concentrations are up to x = 0.9. The L*value which means the lightness of sample is changed around 76. The hue angles (h°) dramatically reduce from 85.09 to 71.86. In other words, the hue angles of the La2Ce2-xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7, 0.9) pigments are found to be in the yellow-orange region of the cylindrical color space (35°–70° for orange, 70°–105° for yellow). Simultaneously, the increase of doping contents results in strengthen of chroma value (C*) which enhances from 23.47 to 39.79. Therefore, the sample of La2Ce1.1W0.45Fe0.45O7+δ with color coordinates of L* = 73.16, a* = 12.39, b* = 37.81 shows a

Fig. 4. UV visible reflection of La2Ce2-xW0.5xFe0.5xO7+δ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9) pigments calcined at 1000 °C. The inset figure shows the diffuse reflectance spectra.

(average reflectance is around 53%) [12,16,33]. As a result, these yellow-orange pigments have shown potentials for the utility as a nontoxic cool pigment. The color coordinates of the La2Ce2-xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7, 0.9) orange pigments calcined at 1000 °C are summarized in Table 3, the parent solid solution oxide La2Ce2O7 (R* = 99.11%, 4

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Table 4 L*, a*, b* color coordinates and NIR solar reflectance for the La2Ce2-xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7, 0.9) pigments calcined at 1150 °C. Sample(1150 °C)

L*

a*

b*

C*

H°

R%

Solar R%

0.1 0.3 0.5 0.7 0.9

78.20 75.72 79.31 77.05 74.51

5.76 11.81 11.30 11.77 11.93

32.44 39.83 40.19 41.38 41.01

32.94 34.52 41.75 41.75 42.71

79.94 70.00 74.30 74.12 73.78

83.27 88.06 89.55 89.22 86.75

78.16 81.29 83.85 82.62 78.76

Table 5 L*, a*, b* color coordinates and NIR solar reflectance for the La2Ce2-xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7, 0.9) pigments calcined at 1300 °C. Sample (1300 °C)

L*

a*

b*

C*

H°

R%

Solar R*%

0.1 0.3 0.5 0.7 0.9

75.92 75.23 74.31 73.68 73.41

8.13 13.34 15.25 14.85 15.12

34.36 38.34 39.81 42.45 43.54

35.30 40.59 42.63 44.97 46.09

76.69 70.81 69.04 70.72 70.85

79.87 82.77 85.30 87.32 87.60

73.04 76.22 77.13 78.78 79.04

Fig. 5. NIR solar reflectance spectra of La2Ce2-xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7, 0.9) pigments calcined at 1000 °C. (NIR reflectance spectra in the inset).

vivid orange hue, which is significantly higher than those of the reported Y2Ce1.85Fe0.15O7 (L* = 76.64, a* = 4.86, b* = 13.19) and Cr2O3-3TiO2 [16,34]. 3.4. Influence of calcination temperature Fig. 7. Photographs of La2Ce2-xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7, 0.9) pigments calcined at 1150 °C.

Associated with XRD analysis, the higher calcination temperature would improve the crystallinity of the sample and in turn influence the color coordinates and optical properties. Fig. 6 illustrates the NIR solar reflectance spectra of La2Ce2-xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7, 0.9) orange pigments calcined at 1150 °C (a) and 1300 °C (b), respectively (their corresponding NIR reflectance spectra were inset). The CIE 1976 color coordinates of the pigment samples calcined at 1150 °C and 1300 °C are list in Table 4 and Table 5. The pigments calcined at 1150 °C increase the NIR solar reflectance to a small degree compared with the sample calcined at 1300 °C, but the color has obvious changes. The color of sample La2Ce1.9W0.05Fe0.05O7+δ changes extremely, specifically in the enhanced redness and yellowness (a* increased from 2.01 to 5.76, and b* increased from 23.75 to 32.44). Meanwhile, the yellowness of all the samples has been improved for the b* value up to around 40. Interestingly, when x > 0.1, the redness of the pigment calcined at 1150 °C are around 11 with very small difference. Fig. 7 shows the photographs of La2Ce2-xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7, 0.9) orange pigments calcined at 1150 °C. When the calcination temperature was raised to 1300 °C, with the

doping of W6+ and Fe3+ for Ce4+, the NIR solar reflectance regularly increases from 73.04% to 79.04%. The NIR solar reflectance values are lower than those calcined at 1150 °C. The chromatic properties have been further improved, the redness (a* values) increases to be around 15 (x > 0.1) and yellowness (b* values) increases gradually from 34.36 to 43.54, the chroma values (C*) of the sample are also higher than those calcined at 1150 °C. In summary, the higher calcination temperature resulted in higher color coordinates and slightly reduced the NIR solar reflectance.

3.5. Application study To verify the practicality of synthesized powder orange pigments for cool coatings, the typical sample La2Ce1.7W0.15Fe0.15O7+δ (calcined at 1150 °C) was coated on a galvanized sheet (a kind of metal sheet roofing material). The weight ratio of pigment sample to alkyd resin

Fig. 6. NIR solar reflectance spectra of La2Ce2-xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7, 0.9) pigments calcined at 1150 °C (a) and 1300 °C (b). (NIR reflectance in the insets).

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deionized water. After drying, the color coordinates of the samples were measured and compared with original pigment. The result color coordinates are listed in Table 6, the color difference (ΔE*) of all the samples is smaller than 0.96, which indicates that the synthesized pigments are chemically stable under the adverse environmental conditions [35]. 4. Conclusions A series of non-toxic cool pigments La2Ce2-xW0.5xFe0.5xO7+δ (x = 0.1, 0.3, 0.5, 0.7, 0.9) have been developed successfully by the Pechini route. The W6+ and Fe3+ co-doped La2Ce2O7 yellow-orange pigments contributed to high NIR solar reflectance. The higher calcination temperature improved the color performance as well as reduced their NIR reflectance slightly. The typical pigment La2Ce1.7W0.15Fe0.15O7+δ (calcined at 1150 °C) was coated on a galvanized sheet and showed a good yellow color (L* = 67.98, a = 16.75, b* = 50.10). In addition, the coating possessed high NIR solar reflectance of 71.01% which is agree with the Chinese architectural standard. The synthesized La2Ce1.3W0.35Fe0.35O7+δ was chemically stable survived under adverse environmental conditions. The developed non-toxic and sustainable yellow-orange pigment can be used as energy-saving coatings in building facade and automotive coating for reducing the interior temperature.

Fig. 8. NIR solar reflectance spectra of bare galvanized sheet and La2Ce1.7W0.15Fe0.15O7+δ pigment. (NIR reflectance spectra and photographs of coatings in the inset).

Table 6 Color coordinates of the La2Ce1.3W0.35Fe0.35O7+δ pigment (calcined at 1150 °C) after acid/alkali resistance test.

Acknowledgements

ΔE*

5% Acid/alkali

L*

a*

b*

a

– HCl HNO3 H2SO4 NaOH

77.05 77.41 77.62 77.94 78.10

11.77 12.13 11.75 11.54 11.59

41.38 41.48 41.01 41.22 41.43

– 0.68 0.52 0.93 0.96

This work was supported by ‘Hundreds Talents Program’ and Science and Technology Service Network Initiative from Chinese Academy of Sciences (Clean and efficient rare earth extraction and recovery technology). References

a

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(binder) was 1:1, appropriate mixture of pigment and binder was grounded for a while to ensure the uniform dispersion of pigment particles. Finally, the above mixture solution was coated on the galvanized sheet and dried in an oven at 90 °C. The NIR solar reflectance spectra of bare galvanized sheet and pigment coating are exhibited in Fig. 8. Similar to our previous research, the coating greatly enhanced the thermal insulation property of galvanized sheet. The bare galvanized sheet surface possessed a low NIR solar reflectance (determined in accordance with ASTM standard G173-03) of 19.18%, and this value was uplifted to be 71.07% with the developed La2Ce1.7W0.15Fe0.15O7+δ pigment coating. The photographs of coatings are also shown in Fig. 8, it can be seen that the chromatic properties of coating have been improved by the alkyd resin compared with its corresponding powder pigment, the lightness (L*) decreases from 75.72 to 67.98, redness (a*) increases from 11.81 to 16.75 and yellowness (b*) uplift from 39.83 to 50.10. Hence the resulting coating shows a more vivid color. According to measurement standard JG/T 235-2014, the NIR solar reflectance of the architectural reflective thermal insulation coating should be greater than L*/100, when the lightness (L*) of coating is in the range of 40–80. Therefore, the developed coating agrees with this standard, which indicates that the synthesized La2Ce1.7W0.15Fe0.15O7+δ pigment can be used as a kind of cool pigment in architecture. 3.6. Acid/alkali resistance studies Inorganic pigments must have a certain acid and alkali resistance in some application scenes. In this study, 5% HCl, HNO3, H2SO4 and NaOH solution were used to test the acid/alkali resistance of the synthesized pigment. The typical pigment of La2Ce1.3W0.35Fe0.35O7+δ (calcined at 1150 °C) was soaked in the acid/alkali and evenly mixed for 10 min. The pigment powder was then filtered and washed with 6

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